The present invention pertains to switch mode power supplies and control thereof, and to the possible use of capacitive sensing to enable and/or improve such control.
ii.a) Standby Consumption of Power Electronics:
Power Electronics and switch mode power supplies have become a mature technology, and finds wide application in today's world. It is notable that almost all portable electronic devices contain some sort of Switch Mode Power Supply (SMPS). A typical example is shown in FIG. 1. Due to the fact that a SMPS typically exhibits high efficiency, it is attractive for mobile or portable electronic devices, as these normally run off batteries. Nonetheless, some losses are still incurred by these efficient switching circuits, which may be anything between a few and multiples of ten percent of the total energy processed and transferred to the load.
Mobile devices that employ SMPS often enter a low power mode after a specific period of inactivity, in order to conserve battery power. However, the SMPS will typically not join this entry into a low power mode, and as such, incur unnecessary losses. Often, the end device being powered by the SMPS has the ability to turn SMPS into low power state by deactivating an enable pin of the SMPS controller. But this action may result in an end device which is not powered anymore and consequently not being able to sense when a user require the system to power up again. Alternatively, the end device may be powered from an exhaustible supply, such as a capacitor, and periodically enable the SMPS when the exhaustible supply runs below a certain level. However, such a scheme may require an unnecessarily large low voltage energy store, and could be prone to reliability issues. Ideally, the microchip controlling the SMPS should have the ability to not only sense when the end device is in an inactive power state, and therefore enter the SMPS into a low power state as well, but also have the ability to sense when the whole system should be powered up again. In other words, it would be ideal if the SMPS controller has some inherent sensing ability for interaction with the user or other devices. An additional challenge is that the power-up sequence needs to be with as little delay as possible. By the time that the user touches the product containing the SMPS, it should be in maximum power transfer mode already. Therefore, some sort of proximity sensing would be beneficial, where a user's approach, or that of a legitimate engaging device, may be sensed, and timely action taken.
ii.b) Charging Mobile Devices:
Another notable aspect of modern life is the large number of portable electronics being charged every day. A typical user has a number of mobile electronic devices, each with its own specific charging requirements, and separate chargers. This results in quite a challenge to keep track of when each mobile device needs to be charged, what charger should be used, and the location of each charger. The prior art contains a number of solutions to address these problems, for instance as disclosed in U.S. Pat. No. 7,952,320 to Research In Motion, but to date, a simple, robust and cost-effective universal charging solution for portable electronic devices is still not widely available, possible due to lack of robust, low cost communication platforms and power consumption challenges.
ii.c) Capacitive Sensing:
Capacitive touch sensing is approaching maturity as a technology. The last two decades have seen wide deployment of capacitive sensing solutions in consumer electronics. However, the underlying premises of the technology is quite old. For instance, the prior art contain reference to a touch enabled lamp in U.S. Pat. No. 2,896,131 that dates back more than five decades. With the recent commercial acceptance of and opportunities for capacitive touch technology, advance has been rapid. The prior art now holds technology that allow detection of user proximity over significant distances, well beyond a few centimeters. Further, a resultant reduction in the cost of integrated capacitive sensing solutions has opened up new applications heretofore untapped. Sensitivity of cost effective capacitive sensing circuits have become high enough to allow accurate sensing even if complimentary dielectric probes or electrodes are not perfectly lined up, and some distance away.
ii.d) Digital Data Transfer:
Another sphere of endeavor which has grown at a tremendous pace in recent decades is digital data transfer, and specifically wireless data transfer. However, a number of challenges still hinder deployment in certain areas. Wireless transceivers tend to be fairly costly components, even in integrated circuit form. Care needs to be taken with relevant legal adherence during design. Emissions from wireless transceivers also tend to travel well beyond that which is intended, due to the nature of the technology, opening up to possibility of illegal reception and demodulation, often for criminal purposes. This necessitates the need for complex encryption of data whenever wireless data transfer is used for secure transactions.
To facilitate data transfer over short distances, such as required in access control, a large number of HF solutions have been developed. Mostly, these are based on inductive energy transfer at a few MHz, used to power an access card, followed by an authentication transaction between reader and card. However, a fair amount of energy is required to transfer the energy inductively. Therefore, the reader can never enter a truly low power state, as it needs to be able to recognize a valid card approach or presence to wake from such a low power state, and this ability requires a fair amount of power.
ii.e) Noise Immunity of Capacitive Sensing:
In capacitive sensing applications, noise may present implementation and utilization challenges, due to the highly sensitive circuits used to sense changes in capacitance. Specifically common mode noise, or noise currents that flow to the electrical earth, can be detrimental to capacitive sensing systems, given that capacitive sensing electrodes always couple with said earth, be it intentional, as with self-capacitance measurements, or unintentional, as with mutual capacitance measurements. International Electromagnetic Compatibility (EMC) standards stipulate immunity to common mode conducted noise in the band from 150 kHz to 80 MHz, or even 230 MHz, if the product is small enough. Especially the lower frequencies of this band coincide with typical charge transfer frequencies used for capacitive sensing, potentially causing problems. If charge is transferred to and from an unknown capacitor such as a touch electrode at only one frequency, common mode conducted noise at this frequency may disrupt the capacitive sensing process, leading to either the false annunciation of touch or proximity events or an inability to sense real touch or proximity events. The prior art have ventured to overcome this obstacle by capacitive sensing techniques based on some spread of the spectrum used. However, the amount of spread that can be realistically realized is limited, both from a hardware perspective, and by sensitivity for human touch, which seems to be optimum at charge transfer frequencies around 500 kHz to 1 MHz. Said techniques also qualify a touch or proximity event for the complete range of frequencies present in the spread, and not for a specific charge transfer frequency per se.
In another technique held by the prior art, [US 2009/031 5850 by Hotelling et al], the use of a number of discrete stimulus frequencies and filtering algorithms are taught to improve multi-touch detection along with noise immunity. The method disclosed by Hotelling et al is a simplified version of the spread spectrum approach. Specifically, the application discloses majority rules, median and average algorithms. In a majority rules algorithm with three discrete frequencies, the measured capacitance value is taken as the average of the two capacitance values which are closest to each other, with the third value seen as possibly corrupted by noise, and discarded of. In a median filter algorithm, the median capacitance value of the three is taken as the measured capacitance. An average filter algorithm is self-explanatory. Once the measured capacitance is obtained from the filter algorithm, it is used to determine whether a touch occurred or not. It should be made clear that said application does not test for a touch at each of the discrete stimulus frequencies, but tests for a touch on a single capacitance value resulting from the three discrete stimulus frequencies. Although the technique disclosed by said application may improve noise immunity, it is also quite possible for noise to actually change the majority rules, median or average filter derived capacitance value in such a way that a touch is falsely detected, and annunciated.
ii.f) Lighting Technology Challenges:
Lighting technology have also seen significant changes in recent times, with focus on energy efficiency relative to light output, as well as on lifetime of lighting elements. Incandescent light bulbs are very close to becoming relics of a bygone age, being first replaced by fluorescent bulbs, and lately by solid state LED bulbs. Given the particular power requirements of these latter two technologies, SMPS are often used with them to convert 50/60 Hz mains power to the correct power frequency and level. Traditionally, lighting fixtures have been switched on and off with wall based, or in-line, high voltage mechanical switch assemblies. However, as high technology proliferate, users are increasingly opting for more intelligent user interfaces, even for lamps. Touch and proximity sensing based user interfaces are ideal candidates for lighting control. However, if prior art SMPS controllers are used to power fluorescent or LED based lighting fixtures, the SMPS will have to be active continuously, to power said intelligent user interfaces. This will inevitably increase power wastage over that of a traditional mechanical switch implementation, negating some of the efficiency gains achieved through the use of LED or fluorescent technology. What is needed is a SMPS controller which do not require the SMPS to be active and switching to be able to power and monitor the intelligent user interface of a lighting fixture or application.
In lighting fixtures of the prior art, another drawback is the absolute nature of control, specifically that executed with round knobs traditionally used to dim light sources, or single touch sensor (TS) dimmers. Users have to move said knobs in exactly the same manner, and in the same direction to obtain the same dimming effect each time. This is also true for applications other than lighting, for instance where knobs are used to control speed of electrical motors, or volume of an audio system etc. Further, prior art control knobs suffer from the fact that they are difficult to locate in the dark, as is often the case when a lamp or lighting fixture needs to be turned on. It would be advantageous if a controlling knob with control parameters which are more relative in nature can be contrived. That is, a control knob which only depends on the end point relative to the starting point to determine control actions. Single TS dimmers suffer from the drawback that the achieved light level is fixed beforehand according to the time that the TS is engaged. That is, if a user touches said TS for a specific time, a specific light level will be achieved. Users have to carefully time their engagement with said TS to obtain a desired light level. This can be an inconvenience. A dimmer control, based on touch detection, with the dimming level independent of the period of user engagement, may improve the art. Having dimmer controllers with some kind of Find-In-The-Dark (FITD) functionality in conjunction with a touch interface may further improve the art.
Lighting prior art also holds a control method whereby the traditional main power switch of the lighting application is used to facilitate a dimming function. Circuitry between said main power switch and the light emitting element are used to detect a number of rapid openings and closures of said switch. A specific level of light is chosen through the execution of a specific number of rapid openings and closures. This method may also be used to select a certain color of light to be emitted. However, said method has the significant drawback that the user has to continuously and rapidly operate said main power switch for a fair number of open/close cycles. This may not be acceptable, from a user convenience point of view, or from a mains power switch lifetime viewpoint.
ii.g) SMPS Audible Noise:
In prior art SMPS supplies, audible noise being emitted by a transformer in the supply, due to switching, may be a nuisance to users. Said noise may also occur in lighting applications where dimming is executed by repetitively keeping the main switching element or elements turned off for one or a number of cycles, reducing the amount of energy transferred by said transformer. The art would benefit from a technique that minimize such audible emissions without a significant increase in the required controller and memory resources.
The invention to be disclosed purports to address the drawbacks of prior art technology as listed above. Specifically, without imposing a limit, it aims to present a possible solution which allows SMPS application with a reduction in overall losses, and a cost effective and robust method to transfer digital data over a barrier consisting of a plurality of dielectrics along with an increased immunity to conducted noise.